Citrine from Zambia
April 2, 2014
Mr Blauwet loaned seven of the cut citrines to GIA for examination, ranging from 3.38 to 43.86 ct (Figure 1). Concave facets on the three largest pieces accentuated their brilliance. The colour of the citrine ranged from pale, slightly brownish yellow to brownish orangey yellow, and the seven stones showed the following gemmological properties: RI— no = 1.542, ne = 1.552; birefringence—0.010; hydrostatic SG—2.65–2.66; UV fluorescence—inert to both long- and short-wave UV radiation, except for one pale yellow sample that fluoresced very weak white under long-wave UV; and no features seen with the desk-model spectroscope. Microscopic examination revealed straight and angular colour zones, as well as a general haziness and bands of hazy particles (Figure 2), which proved natural origin and were similar to those described in citrine from Sri Lanka (E. J. Gübelin and J. I. Koivula, Photoatlas of Inclusions in Gemstones, Vol. 2, Opinion Publishers, Basel, Switzerland, 2005, p. 573). The 3.38 ct stone also contained a plane of parallel tubules and two-phase inclusions (Figure 3). Viewed with cross-polarised light, minor areas of Brazil-law twinning were seen in only the two smallest samples.
FTIR spectroscopy revealed the absence of a 3595 cm–1 band. This feature is sometimes used as confirmation of natural quartz, especially amethyst, when it is well resolved and there are no diagnostic natural inclusions (S. Karampelas et al., “Infrared spectroscopy of natural vs. synthetic amethyst: An update”, Autumn 2011 G&G, pp. 196–201). In citrine this feature is often absent, so proper identification must rely on inclusion observation, or the analysis of trace elements (C. M. Breeding, “Using LA-ICP-MS analysis for the separation of natural and synthetic amethyst and citrine”, www.gia.edu/gia-news-research-nr73109a).
Natural-colour citrine is less common than other quartz varieties such as amethyst and smoky quartz. Assuming that the colouration of this citrine is natural—as represented by the supplier—this large, clean material makes a nice addition to the gem market.
About the Author
Donna Beaton is supervisor of coloured stone services at GIA's New York laboratory.